Turkey has stepped up arms supplies to Syrian rebels to help them stave off an expected offensive by the Syrian army and its Russian and Iran-backed allies in northwest Syria near the Turkish border, rebel sources told Reuters.

Senior rebel officials said Turkey had sent more military aid to rebels in and around the Idlib region since a summit meeting with Iran and Russia last week failed to agree a deal to avert a government offensive into the area.

Turkey, which is already hosting 3.5 million Syrian refugees, warned against such an attack, fearing it could force more Syrians over the border. Turkish President Tayyip Erdogan has warned of a humanitarian disaster and security risks for Turkey.

"They pledged complete Turkish military support for a long, protracted battle," a senior Free Syrian Army commander who was privy to talks in recent days with senior Turkish officials said, requesting anonymity as he was not authorized to speak publicly.

The agency said in a report released Wednesday that over 1 million Syrian men, women and children have been displaced so far this year "with most now living in dire conditions."

Russian and Syrian warplanes have stepped up airstrikes on southern Idlib and adjacent areas of Hama province in an apparent prelude to a ground offensive.

The Syrian army is building up troops near frontlines in preparation for an offensive on the last major rebel stronghold in Syria.

Turkey has backed an array of FSA rebels during the war that spiraled out of an uprising against Syrian President Bashar Assad in 2011. With decisive Iranian and Russian help, Assad has now recovered most of Syria.

Idlib's main towns and cities are under the sway of jihadists linked to al Qaeda's former Syrian affiliate, the Nusra Front, though they are outnumbered by Turkey-backed FSA fighters grouped under "The National Front for Liberation."

The Turkish army has deployed in the last week more troops and heavy weaponry to 12 positions in the Idlib region that observe a "de-escalation zone" agreed with Iran and Russia. The Turkish army has also sent troops into Syrian rebel-held territory further east, in an area north of Aleppo city.

With extensive Turkish support, efforts have been underway to organize FSA groups north of Aleppo into a unified force known as the "National Army" numbering some 30,000 fighters.

Two rebel commanders said Turkey ordered the bulk of this force to move towards the Idlib frontlines.

Tehran summit

At the summit in Tehran, Erdogan, Russian President Vladimir Putin and Iranian President Hassan Rohani agreed in a statement that there could be no military solution to the conflict and it could only end through a negotiated political process.

But while Erdogan had also called for a truce, Putin said this would be pointless as it would not involve the Islamist militant groups that Russia deems terrorists, and Rohani said Syria must regain control over all its territory.

Russia has said Turkey has the job of separating Islamist militants from the moderate opposition in Idlib.

Rebel sources said Turkey had pledged to take strong measures against the jihadists once Russia holds back the Syrian army from waging a major assault.

The jihadists have so far resisted calls by Ankara to dissolve themselves or take an offer that allows fighters to join FSA factions after breaking all ties to Al-Qaida.

The UN Commission of Inquiry warned that a possible offensive on the northwestern province of Idlib "would generate a catastrophic" humanitarian crisis.

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